Characterization of torque-related activity in primary motor cortex during a multijoint postural task.

The present study examined neural activity in the shoulder/elbow region of primary motor cortex (M1) during a whole-limb postural task. By selectively imposing torques at the shoulder, elbow, or both joints we addressed how neurons represent changes in torque at a single joint, multiple joints, and their interrelation. We observed that similar proportions of neurons reflected changes in torque at the shoulder, elbow, and both joints and these neurons were highly intermingled across the cortical surface. Most torque-related neurons were reciprocally excited and inhibited (relative to their unloaded baseline activity) by opposing flexor and extensor torques at a single joint. Although coexcitation/coinhibition was occasionally observed at a single joint, it was rarely observed at both joints. A second analysis assessed the relationship between single-joint and multijoint activity. In contrast to our previous observations, we found that neither linear nor vector summation of single-joint activities could capture the breadth of neural responses to multijoint torques. Finally, we studied the neurons' directional tuning across all the torque conditions, i.e., in joint-torque space. Our population of M1 neurons exhibited a strong bimodal distribution of preferred-torque directions (PTDs) that was biased toward shoulder-extensor/elbow-flexor (whole-limb flexor) and shoulder-flexor/elbow-extensor (whole-limb extensor) torques. Notably, we recently observed a similar bimodal distribution of PTDs in a sample of proximal arm muscles. This observation illustrates the intimate relationship between M1 and the motor periphery.

Kinematics and kinetics of multijoint reaching in nonhuman primates.

The present study identifies the mechanics of planar reaching movements performed by monkeys (Macaca mulatta) wearing a robotic exoskeleton. This device maintained the limb in the horizontal plane such that hand motion was generated only by flexor and extensor motions at the shoulder and elbow. The study describes the kinematic and kinetic features of the shoulder, elbow, and hand during reaching movements from a central target to peripheral targets located on the circumference of a circle: the center-out task. While subjects made reaching movements with relatively straight smooth hand paths and little variation in peak hand velocity, there were large variations in joint motion, torque, and power for movements in different spatial directions. Unlike single-joint movements, joint kinematics and kinetics were not tightly coupled for these multijoint movements. For most movements, power generation was predominantly generated at only one of the two joints. The present analysis illustrates the complexities inherent in multijoint movements and forms the basis for understanding strategies used by the motor system to control reaching movements and for interpreting the response of neurons in different brain regions during this task.